1. Field of the Invention
The invention relates to a process for recovering lecithin from oil- and lecithin-containing mixtures by extraction with an extractant comprising at least one hydrocarbon.
2. Description of the Background
The mixture of phosphatides referred to in the trade as lecithin is a group of phospholipids composed of the following structural members: glycerol, fatty acids, phosphoric acid, and amino alcohols and carbohydrates, respectively. They are found in practically any animal and Vegetable material. Ample amounts thereof are present in brain tissue, egg yolk, and oil seeds such as soybean oil and rape-seed oil.
Phosphatides of vegetable origin are generally highly unsaturated. In lecithin preparations of animal origin, there are found, besides glycerol esters, various sphingolipids and plasmalogens as phosphorus-containing compounds. The main constituents of soybean lecithin are phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl inositol, phosphatidyl serins, phosphatidyl acid, lysophospholipids, and phytoglycolipids.
In vegetable oils from oil seeds and cereal seeds such as soybeans, rape-seed, sunflower seed, corn, hemp and linseed, phosphatides are present in concentrations of from 0.2 to 2% by weight. Also used for the recovery of phosphatides are egg yolk, yeast lipids, and bacterial bio-mass. All of the above materials are suited as a source of phosphatide mixtures in the process of the invention, too. One specific example is the crude lecithin obtained in the refining of vegetable oils and fats.
At present, the major part of commercial lecithine is obtained in the processing of soybean oil and rape-seed oil. They contain lecithin in colloidal from. To the oil, water is added under stirring in accurately measured quantities, and the lecithin contained in the oil is hydrated at elevated temperature. The slimy mixture is separated in a separator and subsequently dried in a drying apparatus under a vacuum. Obtained in this manner is viscous crude lecithin. This crude lecithin recovered by extraction from soybeans or rape-seed in the course of a refining operation consists to about one third of oil and to two thirds of acetone-insoluble components, namely lecithin. For some applications, the oil content is disturbing, and further purification is required.
The term lecithin within the present description does not only mean natural lecithins but also chemically modified lecithins, the latter ones being of increasing interest.
It is state of the art to separate the oil from crude lecithin by extracting with acetone. The powdery product referred to as pure lecithin has a residual oil content of from 2 to 4% by weight. In the course of acetone extraction, there is withdrawn a fat/acetone solution as well as a suspension of phospholipids in acetone. The main part of the acetone is separated from the phospholipids by centrifuging and fed back into the solvent cycle. The partially dried product is then freed of residual solvent in at least two steps. A first drying step is conducted in a fluidized bed drier where the phospholipids are fluidized at about 50.degree. to 70.degree. C. by means of hot air. The main part of acetone is thus removed. To remove even the last traces of solvent, the product is subsequently dried in vacuum drying oven. In this step, thin layers of phospholipids are placed on racks and treated for several hours at elevated temperature (50.degree. to 70.degree. C.). A specific problem of the acetone extraction method are the streams of exhaust air from the drying steps, which may be vented to the environment only after proper purification. Pure lecithin is used not only as an emulsifying food additive but also in undiluted form as a dietetic. Thus, its purity, e.g. freeness of solvent, must meet strict requirements; in the case of acetone with its low threshold value regarding odour, special attention must be given to the above aspect.
The lecithin obtained through acetone extraction must be dried at temperature below 70.degree. C.; otherwise thermal decomposition of the phosopholipids will set in. Even at these temperatures, there will be formed in the course of the drying operation acetone-induced products which impair the organoleptic quality of the de-oiled phospholipids. Described as quality-impairing have been a musty hay-like odour and a sharp pungent aftertaste.
Because of the above described disadvantages of purification with acetone, there have been investigated in recent years alternative methods for the processing of crude lecithin. The complex removal of extractant may be avoided e.g. by using dense gases as solvents.
Disclosed in DE-A 30 11 185 and DE-A 32 29 041 are methods for de-oiling crude lecithin with supercritical dense carbon dioxide and ethane, respectively, wherein the crude lecithin is contacted with the dense gas under conditions that are supercritical with respect to pressure and temperature. Here, the oil preferably dissolves. The loaded gas is transferred from the extracting stage into a regenerating stage where it is separated into gas and extract through variations in pressure and/or temperature. The regenerated gas is fed back into the extracting stage. Since after removal of the major oil portion the lecithin becomes very pasty and, consequently, the material transfer rate is very low, extraction time will be very long. Moreover their Will be required pressures of 35 MPa or more which make the process uneconomical.
To avoid such long extraction times, there was proposed to realize the de-oiling of lecithin by finely distributing the crude lecithin in the carbon dioxide extractant in a nozzle line at brief contact times (DE-A 33 29 249). Obtained here is a powdery, whitish-yellow, oil-free, odourless and tasteless lecithin. The carbon dioxide pressure required for extraction in accordance with the above process is 90 MPa. Due to the high pressure and the low throughput of a nozzle, the process does, however, involve high costs.
Proposed in DE-A 34 11 755 is a process wherein de-oiling of lecithin is accomplished with the aid of dense carbon dioxide to which there has been added a substantial amount of propane as an entraining agent. This procedure has two effects:
(a) the pressure for achieving high loadings of the extractant is reduced to 8 to 12 MPa, and (b) the de-oiled lecithin forms within this pressure range a liquid phase with an extractant composed of 75% of propane and 25% of carbon dioxide, thus permitting countercurrent operation and withdrawal of the de-oiled product from the bottom of the column through a pipe. Carbon dioxide is required so that two phases may form. Under these extracting conditions, the mixture of carbon dioxide/propane is supercritical. The use of an extractant mixture does, however, give rise to additional costs as the composition of the mixture must be continuously controlled and adjusted during the extracting process.
In U.S. Pat. No. 2,548,434 it is suggested a process for the processing of fat containing materials such as oil-seed meals, cracklings and the like. In that process fat is extracted by means of a liquid hydrocarbon which is gaseous at atmospheric pressure. Extraction is conducted at temperatures ranging from 65.degree. to 95.degree. C. Hydrocarbons suited for that process should thus have vapour pressures of greater than 1 atmosphere within this temperature range. The solvent-ratio, expressed in units of volume, should be about 15 to 30. Based on a density of 500 kg/m.sup.3 of the liquid hydrocarbon, this corresponds to a loading of 3 to 6% by weight. Pressures ranging from 35 to 45 bar are employed for an-extraction conducted at temperatures of from 79.degree. to 93.degree. C. Upon the addition of bleaching earth to the starting materials, decolorized oils of high quality are thus obtained. Colour bodies, gums and phosphatides remain with the residual materials of vegetable or animal origin.
Suited for that process are solvents at temperatures ranging from about 60.degree. C. below their critical temperature up to a few degrees above said critical temperature. If that extraction process is used for the processing of crude oils two fractions will be obtained, one of these fractions being free of colour bodies, gums and phosphatides while the other one is enriched in phosphatides besides colour bodies and gums. The phosphatides which are present in the starting material in concentrations of up to 0.5% by weight may be thus enriched to concentrations of 3 to 5% by weight.
In contrast thereto, the process for the preparation of pure lecithin according to the present invention starts out from crude lecithin containing phosphatides in concentrations of from 50 to 70% by weight. In the said U.S. Pat. No. 2,548,434, on the other hand, there is disclosed no way of obtaining pure lecithin from crude lecithin.